minor #9429 Add rule to import classes except for built-in PHP classes. (umulmrum)

This PR was squashed before being merged into the 2.7 branch (closes #9429).

Discussion
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Add rule to import classes except for built-in PHP classes.

Fixes #9426 (class imports were shown in the example but there was no rule in the text).

Commits
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9c75279 Add rule to import classes except for built-in PHP classes.

Author: javiereguiluz

contributing/code/standards.rst

@@ -192,7 +192,9 @@ Structure
 * Do not use ``else``, ``elseif``, ``break`` after ``if`` and ``case`` conditions
   which return or throw something;
 
-* Do not use spaces around ``[`` offset accessor and before ``]`` offset accessor.
+* Do not use spaces around ``[`` offset accessor and before ``]`` offset accessor;
+
+* Add a ``use`` statement for every class that is not part of the global namespace.
 
 Naming Conventions
 ~~~~~~~~~~~~~~~~~~